The present invention relates generally to the compression of a refrigerant gas into which a liquid has been injected during the compression process. More particularly, the present invention relates to the requirement to separate entrained oil from the refrigerant gas-oil mixture discharged from an oil-injected screw compressor in a refrigeration system.
Compressors used in refrigeration systems raise the pressure of a refrigerant gas from a suction to a discharge pressure thereby permitting the refrigerant to be used within the system to cool a desired medium. Many types of compressors, including rotary screw compressors, are employed to compress the refrigerant gas in such systems.
In a screw compressor, two complimentary rotors are located in the working chamber of a rotor housing. The housing has a low pressure end defining a suction port and a high pressure end defining a discharge port. Refrigerant gas at suction pressure enters the low pressure end of the compressor's working chamber through the suction port and is enveloped in a pocket formed between the counter-rotating screw rotors. The volume of the gas pocket decreases and the pocket is circumferentially displaced to the high pressure end of the working chamber as the compressor rotors rotate and mesh.
The gas within such a pocket is compressed and heated by virtue of the decreasing volume in which it is contained prior to the pocket's opening to the discharge port. The pocket, as decreases in volume, eventually opens to the discharge port at the high pressure end of the rotor housing where the compressed gas is discharged.
Screw compressors used in refrigeration applications will, in the majority of instances, include an oil injection feature. Oil is injected into the working chamber in relatively large quantity (and therefore into the refrigerant gas being compressed) for several reasons. First, the injected oil acts to cool the refrigerant gas undergoing compression. As a result, the intermeshed rotors are cooled allowing for tighter tolerances between them.
Second, the oil acts as a lubricant. One of the two rotors in a screw compressor is typically driven by an external source such as an electric motor. The mating rotor is driven by virtue of its meshing relationship with the externally driven rotor. The injected oil prevents excessive wear between the driving and driven rotors and is additionally delivered to various bearing surfaces within the compressor for lubrication purposes.
Finally, oil injected into the working chamber of a screw compressor acts as a sealant between the rotors themselves and between the rotors and the wall of the working chamber for the reason that there are no discrete seals between the individual rotors or between the rotors and the rotor housing. Absent the injection of oil, significant leakage paths would exist which would be detrimental to compressor efficiency. Oil injection therefore both increases the efficiency and prolongs the life of the screw compressor.
Oil making its way into the working chamber of a screw compressor is, for the most part, atomized and becomes entrained in the refrigerant undergoing compression. Such oil must, to a great extent, be removed from the oil-rich refrigerant gas discharged from the compressor in order to make the oil available for return to the compressor for the purposes enumerated above. Further, removal of excess oil from the compressed refrigerant gas must be accomplished to ensure that the performance of the gas is not unduly affected within the refrigeration system by the carrying of an excess amount of oil into and through the system heat exchangers.
Previous apparatus by which cyclonic oil separation is accomplished is illustrated in U.S. Pat. Nos. 4,070,168; 4,506,523 and 5,029,448. The latter is assigned to the assignee of the present invention.
The '168 patent is directed to a liquid-gas separator the function of which is to separate entrained gas from a liquid such as crude oil through the use of a scroll housing. The path followed by the mixture within the separator is of ever increasing width.
The '523 patent is thought to be typical of many previous "cyclone"-type oil separators. It is, however, quite distinct from the oil separator of the present invention, as will come to be understood, by its use of concentric discharge gas conduit. In the case of the '523 patent, the discharge conduit through which the separated refrigerant gas exits the separator is concentric with the separator housing and is of enlarged diameter interior of the separator housing.
Finally, the '448 patent, like the '523 patent, is, with respect to individual ones of its legs, suggestive of the earlier known and commonly used cyclone-type oil separators in which a vortex action is induced into a fluid stream for oil separation purposes. The separator of the '448 patent is unique with respect to its separation of the inlet mixture into two discrete streams for delivery to different locations within the separator where centrifugal oil separation processes occurs.
The need continues to exist for a more efficient "cyclonic" oil separator which is particularly suited for use with refrigeration compressors of the screw type.